drm/nouveau: get vmm via nouveau_cli_vmm()

Provide a getter function for the client's current vmm context. Since
we'll add a new (u)vmm context for UMD bindings in subsequent commits,
this will keep the code clean.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804182406.5222-5-dakr@redhat.com